As a nuclear medicine imaging apparatus, Positron Emission computed Tomography (PET) apparatuses are conventionally known. A PET apparatus generates, for example, a function image of a tissue in a human body. More specifically, to perform an image taking process using a PET apparatus, a subject is first dosed with a radiopharmaceutical labeled with a positron emitting nuclide. After that, the positron emitting nuclide that is selectively taken into a body tissue within the subject releases positrons, and the released positrons are coupled with electrons and annihilated. At this time, the positrons release a pair of gamma rays in substantially opposite directions. The PET apparatus detects the gamma rays by using a detector arranged in a ring formation so as to surround the subject and generates simultaneous count information (hereinafter, a “coincidence list”) from the detection result. Further, the PET apparatus performs a reconstructing process through a back-projection process by using the generated coincidence list and generates a PET image.
In this situation, the radiopharmaceutical refers to a pharmaceutical in which a radio isotope (RI) is used. During an image taking process using a PET apparatus, it is required to accurately understand the radioactivity level of the radiopharmaceutical with which the subject is dosed. However, because the radioactivity level of a radiopharmaceutical decays over the course of time (see FIG. 12), the radioactivity level is usually understood by using a test time indicating the time at which the radioactivity level of the radiopharmaceutical is measured and a detection time indicating a time at which gamma rays are detected by a PET apparatus. For example, a time difference is calculated between the test time measured by a radioactivity measuring apparatus and the detection time at which the gamma rays are detected by the PET apparatus, so that the radioactivity level at the time of the detection of the gamma rays can be estimated by referring to a decay curve while using the calculated time difference and the radioactivity value at the test time. FIG. 12 is a drawing for explaining the decay of the radioactivity level of a radiopharmaceutical.
However, the clock used by the radioactivity measuring apparatus to measure the test time is different from the clock used by the PET apparatus to measure the detection time. For this reason, if the clock used by the PET apparatus is not accurate, for example, it means that the detection time itself is not accurately measured. Thus, even if the time difference between the test time and the detection time is calculated, the calculated time difference is inaccurate. As a result, conventional techniques have a problem where it is not necessarily always possible to accurately understand the radioactivity level of the radiopharmaceutical with which the subject is dosed.